Chemical crosslinking is an effective way to deal with solvent and heat susceptibility of polymeric nano-/submicrostructures manufactured by electrospinning. From a very simple point of view, the crosslinking can be considered as an irreversible interconnection of polymer chains by either covalent or ionic chemical bonds. Cross-linking of electrospun nano-/submicrostructures is desired mainly for obtainment of robust products which resists against glass-transition or melting-like effects in elevated temperatures, and also against swelling or dissolving in solvents. Thus, undesired morphological deviations of electrospun structures above polymer-specific temperature limits (mainly Tg), along with susceptibility against solvents can be avoided by cross-linking the polymeric material constituting the product.
Bonding between individual polymer chains via crosslinking restricts their movement and causes an inherent increase in the viscosity. This is unfavorable in polymer processing applications as they typically require shear mixing which demands mobility of the polymer chains. In electrospinning, individual polymer chains in a polymer solution are drawn as nano- or submicroscaled structures in form of fibers under high voltage typically through a nozzle/orifice with a very small diameter (˜1 mm), and high viscosities in electrospinning mixtures render the flow through such narrow nozzle more difficult i.e. more energy-consuming. This concern applies also for needleless electrospinning methods e.g. free-surface electrospinning, sharp edge or pointed tip electrospinning etc.; since electrospinning methods generally comprise releasing of an electrospinning mixture through a release means which can be a nozzle, an orifice, a needle etc. or from an open surface which can be of cylindrical, spherical, wire, pin and slot type electrodes etc.
Typically, a constant flow rate is desired for an uninterrupted flow of an electrospinning mixture for obtaining continuous nano-/submicrostructures therefrom. Thus during electrospinning, fluidic properties of the electrospinning mixtures, especially its viscosity, are typically required to be constant/stable within certain limits for desired form of deposition of the electrospinning mixture, such as continuous fiber form. External effects such as fast solvent evaporation and/or crosslinking throughout the electrospinning process increase the viscosity of the electrospinning mixture and restrain the electrospinning process.
Chemical cross-linking can be performed externally on mats of electrospun nano-/submicrostructures by exposure of the mats to a cross-linking medium (as in ex-situ methods addressed in the below Table 1). Alternatively, cross-linking can be initiated in-situ by introducing cross-linking agents to polymer solutions prior to electrospinning. Ex-situ cross-linking herein can be classified as more conventional and as a direct way to achieve cross-linking by permanently changing the chemistry of nano-/submicrostructures. Whereas, in situ cross-linking methodology is relatively new and is a more controllable bulk cross-linking technique in comparison with other techniques. In-situ cross-linking requires an initiation event (by providing energy onto the polymer in form of heat, UV etc.) that is tunable according to the type of application by choosing a correct composition of polymer mixture to be used in electrospinning.
Main potential problems driven by premature cross-linking in the polymer solution (i.e. cross-linking initiated and/or accelerated before or during the electrospinning) are interruption of flow continuity of an electrospinning mixture through a nozzle (or any respective release means as mentioned above) and choking of solidified polymer in the nozzle. In other words, regardless of the electrospinning method, the electrospinning of polymer solutions containing any crosslinking agents is typically limited with the crosslinking reaction time.
Thus, it is necessary to provide a method for obtaining of an electrospinning composition which maintains a desired viscosity even after several hours from preparation of thereof. The term ‘desired viscosity’ refers to viscosity values suitable for electrospinning. It is further necessary to provide a method for obtainment of heat and solvent resistant, stimuli-self-crosslinkable nano-/submicrostructures.
TABLE 1Benchmarking of state-of-the-art techniques for cross-linking,and their main drawbacks.Cross-linking typeDrawbacksEx-situExposing an electrospun fiber mat to a fluidTime consuming.cross-linking medium (liquid or vapor), orCauses substantialspraying a cross-linking agent thereonmorphological changes.In-situProcesses requiring anUsing an UV-Restricted with UV-curableadditional set-uplight sourcepolymers.Requires additionalequipment.Using a dual-Additional viscositysyringe reactivemodifiers and removal ofcross-linkingthem.set-upTime consuming.Post-electrospinning treatmentHeat treatmentCuring temperaturerestrictions and relatedmorphological changes basedon the glass-transitiontemperature (Tg) of thepolymer.Single step in-situ cross-linkingViscosity changes duringelectrospinning.Time-dependent procedure.